What is a Biosand Filter?
A biosand filter (BSF) is an adaptation of the traditional slow sand filter, which has been used for community drinking water treatment for 200 years. The biosand filter is smaller (about 1 m tall, 0.3 m wide on each side) and adapted so that it does not flow continuously, making it suitable for use in people’s homes. The filter container can be made of concrete or plastic. It is filled with layers of specially selected and prepared sand and gravel. The sand removes pathogens and suspended solids from contaminated drinking water. A biological community of bacteria and other micro-organisms grows in the top 2 cm of sand. This is called the biolayer. The micro-organisms in the biolayer eat many of the pathogens in the water, improving the water treatment.
Pathogens – micro-organisms in water that make us sick
Suspended Solids – dirt and other small pieces in the water (may also be called “turbidity”)
How Does the Biosand Filter Work?
You can use any kind of water in the biosand filter – well water, borehole water, pond or river water, tap-stand water, or rainwater. This makes it very convenient for people because they can use whichever water source is closest to home, make it safe to drink. The water must not have been chlorinated though, or the chlorine will kill the biolayer. The water should also not contain any dangerous chemicals, because the biosand filter cannot remove most chemicals from water.
Contaminated water is poured into the top of the biosand filter at least once per day (but not continuously). The water poured into the top of the filter slowly drips through the holes in the diffuser, and flows down through the sand and gravel. Treated water flows out of the outlet tube. No power is required - the filter works by gravity. It should take about 1 hour to get 12-18 litres of filtered drinking water.
Pathogens and suspended solids are removed through biological and physical processes that take place in the sand. These processes include: mechanical trapping, predation, adsorption, and natural death.
The biosand filter has been studied in the field and in labs. It has been shown to remove the following from contaminated water:
The filter can also remove up to 95% of turbidity (dirt and cloudiness), and up to 95% or iron (which people often don’t like because it turns water, laundry and food red!). Like other filters, the biosand filter cannot remove dissolved contaminants or chemicals, such as salt, arsenic or fluoride. There is an adaptation of the biosand filter using rusty nails, called the Kanchan Filter, which can remove arsenic from water. CAWST has prepared a summary of laboratory and field studies for the biosand filter:
The biosand filter is best used as one step in a multi-barrier approach to safe drinking water. Different treatment methods remove different things from contaminated water. Although the water may look clear after filtration, there may still be some bacteria and viruses in the water. It is necessary to also disinfect the filtered water to ensure the safest drinking water possible. The most common low-cost methods used around the world to disinfect drinking water are:
Chlorine (liquid or tablet) Solar disinfection (SODIS) Boiling
History of the Biosand Filter
Dr. David Manz developed the household biosand filter in the 1990s at the University of Calgary, Canada. In 2001, CAWST was founded to provide the professional services needed for the humanitarian distribution of the filter in developing countries. As new research becomes available, CAWST periodically updates the biosand filter design and recommended operating conditions to provide better water treatment. The current recommended filter is Version 10.
As of 2013,CAWST estimates that well over 650,000 biosand filters have been implemented in 55+ countries around the world, benfitting more than four million people.
How Can I Learn More About the Biosand Filter?
Training is essential to ensure the proper and consistent use of the filters in diverse regions of the world. CAWST offers workshops around the world to train individuals, organizations, and government agencies on how to implement biosand filter projects. Workshops will help prepare you to promote, construct, install, and monitor biosand filters, as well as how to educate users about filter operation and maintenance. CAWST’s workshops are delivered in a participatory way, so that you will remember what you learn and come away with the hands-on knowledge you need to implement a biosand filter project in a developing country.
CAWST also serves as a centre of expertise, providing technical and implementation support in project development, troubleshooting, education resources, network building, and project management for household water treatment, sanitation and hygiene. Please contact us at firstname.lastname@example.org if you have any questions.
Biosand Filter Knowledge Base
www.biosandfilters.info is an online Knowledge Base for Biosand Filter implementers. Join the biosand filter community and share your knowledge and experience.